1. Field of the Invention
The invention relates to charge/discharge control apparatus, method and program for controlling the charging and discharging of a secondary battery formed by a battery pack constituted of a plurality of unit batteries, and a battery control system.
2. Description of the Related Art
Hybrid vehicles equipped with an electric power generator driven by an engine, in addition to a vehicle-driving electric motor, are known. In such a hybrid vehicle, a battery pack formed by connecting a plurality of unit batteries (e.g., battery cells, battery blocks, etc.) in series is mounted. The battery pack drives the vehicle-driving motor by supplying electric power to the motor, and is charged with electric power from the generator (that, in some cases, serves as a vehicle-driving motor as well).
FIG. 5 shows an example of the construction of a battery control system 1100 for the charge/discharge control of a battery pack as described above. In FIG. 5, a battery pack 112 in which a plurality of battery cells 110 are connected in series is connected to a voltage detector 114 provided for detecting the voltage of each battery cell 110. On the basis of the output of the voltage detector 114, a battery ECU 116 computes a value regarding the state of charge (SOC) of each battery cell 110. The value of voltage detected by the voltage detector 114 may be the value of voltage of each battery cell 110 as described above. Furthermore, since the battery pack 112 has a series-connected arrangement of a plurality of battery blocks each of which is formed by connecting a plurality of battery cells 110 in series, the value of voltage detected by the voltage detector 114 may be the value of voltage of each battery block. In this case, the battery ECU 116 computes the SOC of each battery block from the value of voltage of the battery block. On the basis of the thus-computed SOC value, an HVECU 118 controls loads 120 that include a vehicle-driving electric motor, a generator-driving engine, a generator, an inverter, etc., and performs control of adjusting the amount of charge or discharge of the battery pack 112 so that the SOC value of the battery pack 112 is within a predetermined range. A typically adopted control range of SOC is, for example, the range of 20% to 80%. An SOC control method for the battery pack 112 as described above is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2000-14029.
If, in the foregoing related-art control method, the SOC of the battery pack 112 is controlled so as to be within a predetermined range, for example, the range of 20% to 80%, the lower limit value (20%) and the upper limit value (80%) of the control range provide different representative values of SOC of the battery pack 112 as indicated in FIGS. 6A and 6B (where the vertical axis indicates the number of unit batteries of a battery pack and the horizontal axis indicates the SOC of each unit battery). That is, if the SOC of the entire battery pack 112 is controlled with reference to the lower limit value of the control range, the control is performed as follows. That is, the remaining capacity of each battery cell 110 or each battery block is determined from the voltage thereof. The minimum value among the remaining capacities of the battery cells or battery blocks is determined as a representative SOC for the control of the SOC of the entire battery pack 112, that is, the control is performed so that the representative SOC does not fall below the lower limit value. If the SOC of the entire battery pack 112 is controlled with reference to the upper limit value of the control range, the control is performed as follows. That is, the maximum value among the remaining capacities of the individual battery cells 110 or the individual battery blocks is determined as a representative SOC of the battery pack 112, and the control is performed so that the representative SOC does not exceed the upper limit value.
The variation among the charge/discharge capabilities of the individual battery cells 110 increases with increasing time of use of the battery pack 112. That is, during an initial period of use of the battery pack 12, the variation in SOC among the battery cells 110 or the battery blocks is small as indicated in FIG. 6A. However, as time elapses, the variation in SOC increases as indicated in FIG. 6B. According to the above-described control method, however, since the minimum and maximum values of the varying SOC values are used for the control of the lower and upper limits of the control range, the SOC values of all the battery cells 110 or all the battery blocks are always within the control range.
The method disclosed in Japanese Patent Application Laid-Open Publication No. 2000-14029 has a problem of complicated computation for the SOC control since the SOC is controlled with reference to the lower and upper limit values of the control range. In order to solve this and other problems, the present inventors have proposed a battery pack charge/discharge control method capable of carrying out the SOC control through simple computation (Japanese Patent Application No. 2002-43216). The method of the patent application No. 2002-43216 is characterized in that the minimum value of the capacities of unit batteries (e.g., battery cells or battery blocks) constituting a battery pack is computed, and the minimum capacity value is set as a representative SOC, that is, as a reference value, and the representative SOC (control state-of-charge charge) is used for the control.
However, according to the control method of the aforementioned patent application, it sometimes becomes impossible to increase the representative SOC to a control center value if the capacity variation among the unit batteries (capacity differences among the unit batteries) becomes great (e.g., if the capacity variation rapidly increases due to occurrence of an abnormality or the like within the battery in addition to an increase in the capacity variation resulting from the normal use of the battery).
Such a case is exemplified in FIGS. 7A and 7B (where the vertical axis indicates the number of unit batteries of a battery pack and the horizontal axis indicates the SOC of each unit battery). FIG. 7A indicates a case where the capacity variation among the unit batteries of the battery pack is small whereas FIG. 7B indicates a case where the capacity variation has become great. Since the unit battery having the maximum capacity value in the battery pack is restricted in the charging operation by an upper limit value, the upper limit value of the minimum capacity value among the unit batteries, that is, the upper limit value of the representative SOC, decreases with the increasing variation in capacity. Therefore, when the movable width of the representative SOC reduces due to decreases in the representative SOC (upper limit value) and the capacity variation increases to or beyond a certain extent, it becomes impossible to increase the representative SOC to the control center value (FIG. 7B). If the representative SOC cannot be increased to the control center value, there occurs an inconvenience in which even though the battery pack is actually charged to a considerable extent, it is determined that recovery to a sufficiently charged state has not been accomplished.
Such a case will be described with reference to a hybrid electric vehicle illustrated in FIG. 5. When the representative SOC increases to the control center value, the battery ECU 116 determines that the battery pack 112 has been sufficiently charged. Until an indication of the determination result is transmitted to the HVECU 118, the HVECU 118 continues commanding the load 120 to charge the battery pack 112. That is, the HVECU 118 performs a control of sending a command to the load 120 so as to drive the engine so that engine power is allocated for the charging of the battery pack by the generator in addition to the driving of the vehicle by the vehicle-driving motor. However, when the capacity variation becomes considerably great, the representative SOC (upper limit value) no longer increases to reach the control center value (FIG. 7B). Then, even though the battery pack is considerably charged, the battery ECU 116 determines that the representative SOC has not increased to the control center value. Therefore, the battery ECU 116 does not output to the HVECU 118 an indication that the SOC has increased to the control center value, and the HVECU 118 does not command the load 120 to stop the charging operation. If this event happens, the charging of the battery pack does not stop but inconveniently continues. In some cases, for the continued charging, the engine cannot be stopped. In some other cases, a hunting phenomenon of repetitive alternation between the charging and the stop thereof may occur. Furthermore, during a run of the vehicle, as it is determined that charging is incomplete despite accomplishment of practically maximum charge, the engine power is consumed for the charging of the battery pack 112 by the generator in addition to the driving of the vehicle by the vehicle-driving motor. Hence, there occurs a case where during a run of the vehicle that requires increased power, for example, an uphill run or the like, sufficient energy cannot be supplied for the driving of the vehicle, thus failing to meet a drivability requirement.